Recently, there has been a trend toward power consumption reduction of electrical appliances, particularly with battery-operated apparatuses depending on environmental concerns. Generally, means for power consumption reduction requires two important things. One is to reduce power consumed by electrical appliances. The other is to improve efficiency of the power supply itself and minimize wasteful power consumption.
One method for reducing power consumed by an apparatus includes dropping a voltage supplied to the apparatus. Recent apparatuses are tending to become multi-functional and various types of circuits are activated at various operation modes depending on the usage. In addition, another method includes dropping a voltage of the apparatus in response to various kinds of circuits. A power supply voltage meeting such performance requirements varies. Generally, to avoid a complicated power supply circuit, the power supply circuits are set for a circuit that requires the highest voltage. Such power supply circuits also need to supply the voltage to a circuit that satisfactorily performs even with a low voltage, resulting in the consumption of wasteful electric power.
On the other hand, commonly available direct current power sources include a switching regulator and a series regulator. The switching regulator provides a high efficiency in a rated load, high ripple of an output voltage and a high noise during operation, and relatively high power consumed by the regulator, thereby significantly reducing efficiency when a load consumes less power. Further, the switching regulator has a slightly slow response time in response to a rise time, input voltage fluctuations, and load variations so that an output voltage stability is limited.
Meanwhile, with large load current, the series regulator provides a low efficiency due to a high power consumed by a control transistor, low ripple of an output voltage, and low noise during operation. In addition, the series regulator can reduce power consumed by an internal power source control circuit. Thus, with small load current, the series regulator is often more efficient than the switching regulator. Further, the series regulator has a quick response time in response to a rise time, input voltage fluctuations, and load variations, thereby increasing output voltage stability.
In this way, the direct current power sources have the above-mentioned types of regulators and make the most of each regulator's characteristics to reduce electric power. Conventionally, various power sources having these two regulator's characteristics are suggested. One example is described in Japanese Laid-Open Patent Application Publication No. 11-3126. FIG. 1 illustrates a DC-DC converter as described in the above-mentioned publication.
In FIG. 1, IN1 and IN2 are input terminals at which a voltage Vi (e.g., 5 volts) is supplied from a direct current power source (not shown). OUT1 and OUT2 are output terminals for outputting a predetermined direct current power source Vo (e.g., 3 volts). A switching regulator 101 and a series regulator 102 are connected in parallel between these input terminals and output terminals, respectively. An output voltage of the series regulator 102 is set slightly lower than the output voltage (e.g., 2.95 volts) of the switching regulator 101.
When the output voltage generated by the slow-responding switching regulator 101 drops from 3 volts to 2.95 volts or lower due to input voltage fluctuations and load variations, the quick-response series regulator 102 starts its control operation so as to maintain an output voltage Vo at 2.95 volts. When the output voltage Vo again rises to be higher than 2.95 volts, the series regulator 102 is deactivated, decreasing the consumed voltage in the series regulator 102. In this way, the switching regulator 101 can achieve a quick response speed from the switching regulator 102 while maintaining high efficiency.
However, in the configuration described in Japanese Patent Laid-Open Application Publication No. 11-3126, response characteristics of the power source is only improved. With a low output voltage, the series regulator 102 instead of the switching regulator 101 is not used and continues to activate the switching regulator 101 having significantly decreased efficiency, resulting in the consuming of wasteful electric power. Moreover, under certain operating conditions in certain kinds of loads, a decrease in the output voltage can further decrease the electric power consumption. Such considerations, however, are not made.
Alternatively, to reduce the electric power consumption of the apparatus, electric power including a plurality of voltages is supplied to the apparatus as a power source voltage to meet performances of the load mounted on the apparatus. As shown in FIG. 2, for example, a system apparatus 120 includes a first load 121 and a second load 122. The first load 121 receives a predetermined constant voltage VoA from a switching regulator 111 as a power source voltage. The second load 122 receives a predetermined constant voltage VoB from a series regulator 112 as a power source voltage. The first load 121 operates at greater power source voltage than the second load 122 so that the switching regulator 111 and the series regulator 112 output predetermined constant voltages, respectively, so as to provide a formula VoA>VoB.
In this case, the second load 122, which operates at a low power source voltage, generally includes a protection circuit to avoid leakage current from the first load 121 which operates at a high power source voltage. In order to reduce electric power consumption of the system apparatus 120, one approach is that the power source voltage in the first load 121 and the second load 122 is also reduced depending on the usage. For example, when the constant voltages VoA and VoB which are each power source voltage of the first load 121 and the second load 122 are decreased, the constant voltage VoA should remain above the constant voltage VoB, which is the power source voltage of the second load 122. To eliminate such conditions, the protection circuit is required to be mounted on the first load 121 and the second load 122 to avoid leakage current, thereby increasing the circuit scale.